; time arithmetic
(define (julian year month day)
(let* ((a (quotient (- 14 month) 12))
(y (+ year 4800 (- a)))
(m (+ month (* 12 a) -3)))
(+ day
(quotient (+ (* 153 m) 2) 5)
(* 365 y)
(quotient y 4)
(- (quotient y 100))
(quotient y 400)
(- 32045))))
(define (gregorian julian)
(let* ((j (+ julian 32044))
(g (quotient j 146097))
(dg (modulo j 146097))
(c (quotient (* (+ (quotient dg 36524) 1) 3) 4))
(dc (- dg (* c 36524)))
(b (quotient dc 1461))
(db (modulo dc 1461))
(a (quotient (* (+ (quotient db 365) 1) 3) 4))
(da (- db (* a 365)))
(y (+ (* g 400) (* c 100) (* b 4) a))
(m (- (quotient (+ (* da 5) 308) 153) 2))
(d (+ da (- (quotient (* (+ m 4) 153) 5)) 122))
(year (+ y (- 4800) (quotient (+ m 2) 12)))
(month (+ (modulo (+ m 2) 12) 1))
(day (+ d 1)))
(values year month day)))
(define (julian-time year month day hour minute second)
(+ (julian year month day)
(/ (+ (* hour 60 60) (* minute 60) second) 86400)))
(define (gregorian-time time-stamp)
(let* ((date (floor time-stamp))
(time (* (- time-stamp date) 86400))
(hour (quotient time 3600))
(minute (modulo (quotient time 60) 60))
(second (modulo time 60)))
(call-with-values
(lambda () (gregorian date))
(lambda (year month day)
(values year month day hour minute second)))))
(display (julian-time 2015 9 14 10 39 42)) (newline)
(call-with-values
(lambda () (gregorian-time (julian-time 2015 9 14 10 39 42)))
(lambda (year month day hour minute second)
(display year) (newline)
(display month) (newline)
(display day) (newline)
(display hour) (newline)
(display minute) (newline)
(display second) (newline)))